The present invention relates to torque sensors and, more particularly, to non-contacting magnetoelastic torque sensors for providing a measure of the torque transmitted radially in a disk-shaped member.
In the control of systems having rotating drive shafts, torque and speed are the fundamental parameters of interest. Therefore, the sensing and measurement of torque in an accurate, reliable and inexpensive manner has been a primary objective of workers for several decades.
Previously, torque measurement was accomplished using contact-type sensors directly attached to the shaft. One such sensor is a xe2x80x9cstrain gaugexe2x80x9d type torque detection apparatus, in which one or more strain gauges are directly attached to the outer peripheral surface of the shaft and a change in resistance caused by strain is measured by a bridge circuit or other well known means. However, contact-type sensors are relatively unstable and of limited reliability due to the direct contact with the rotating shaft. In addition, they are very expensive and are thus commercially impractical for competitive use in many of the applications, such as automotive steering systems, for which torque sensors are now being sought.
Subsequently, non-contact torque sensors of the magnetostrictive type were developed for use with rotating shafts. For example, U.S. Pat. No. 4,896,544 to Garshelis discloses a sensor comprising a torque carrying member, with an appropriately ferromagnetic and magnetostrictive surface, two axially distinct circumferential bands within the member that are endowed with respectively symmetrical, helically directed residual stress induced magnetic anisotropy, and a magnetic discriminator device for detecting, without contacting the torqued member, differences in the response of the two bands to equal, axial magnetizing forces. Most typically, magnetization and sensing are accomplished by providing a pair of excitation or magnetizing coils overlying and surrounding the bands, with the coils connected in series and driven by alternating current. Torque is sensed using a pair of oppositely connected sensing coils for measuring a difference signal resulting from the fluxes of the two bands. Unfortunately, providing sufficient space for the requisite excitation and sensing coils on and around the device on which the sensor is used has created practical problems in applications where space is at a premium. Also, such sensors appear to be impractically expensive for use on highly cost-competitive devices, such as in automotive applications.
Most recently, torque transducers based on measuring the field arising from the torque induced tilting of initially circumferential remanent magnetizations have been developed which, preferably, utilize a thin wall ring (xe2x80x9ccollarxe2x80x9d) serving as the field generating element. See, for example, U.S. Pat. Nos. 5,351,555 and 5,520,059 to Garshelis. Tensile xe2x80x9choopxe2x80x9d stress in the ring, associated with the means of its attachment to the shaft carrying the torque being measured establishes a dominant, circumferentially directed, uniaxial anisotropy. Upon the application of torsional stress to the shaft, the magnetization reorients and becomes increasingly helical as torsional stress increases. The helical magnetization resulting from torsion has both a circumferential component and an axial component, the magnitude of the axial component depending entirely on the torsion. One or more magnetic field vector sensors sense the magnitude and polarity of the field arising, as a result of the applied torque, in the space about the transducer and provides a signal output reflecting the magnitude of the torque. Inasmuch as the peak allowable torque in a ring sensor is limited by slippage at the ring/shaft interface, concerns have been expressed regarding distortion arising from slippage at the ring/shaft interface under conditions of torque overload. This, together with the need for multiple parts of different materials to minimize the adverse effects of parasitic fields, have encouraged the investigation of alternative constructions.
Most recently, magnetoelastic torque transducers have been developed in which the active, torque sensing region is formed directly on the shaft itself, rather than on a separate ferromagnetic element which then has to be affixed to the shaft. See, for example, PCT International Publication Nos. WO 99/21150 and WO 99/21151. In one form of these newly developed transducers, the magnetoelastically active region is polarized in a single circumferential direction and possesses sufficient magnetic anisotropy to return the magnetization in the region, following the application of torque to the member, to the single circumferential direction when the applied torque is reduced to zero. The torqued shaft is desirably formed of a polycrystalline material wherein at least 50% of the distribution of local magnetizations lie within a 90xc2x0 quadrant symmetrically disposed around the direction of magnetic polarization and have a coercivity sufficiently high that the transducing region field does not create parasitic magnetic fields in proximate regions of the shaft of sufficient strength to destroy the usefulness, for torque sensing purposes, of the net magnetic field seen by the magnetic field sensor. In particularly preferred forms of such transducers the shaft is formed of a randomly oriented, polycrystalline material having cubic symmetry and the coercivity is greater than 15, desirably greater than 20 and, preferably, greater than 35.
In all of the non-contact magnetoelastic torque transducers developed to date, the transducer element has been disposed axially along a shaft used to transmit torque between axially separated locations on the shaft. However, in many applications, axial space is severely limited and/or torque is inherently being transmitted between radially separated locations, e.g., from a shaft to a rim or vice versa, as in a gear, pulley, chain sprocket, and the like. The need to accurately and non-contactingly sense torque in such devices has not heretofore been addressed.